A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps:
1. Melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass. PA1 2. Forming an article from the glass and cooling the glass below its transformation range. PA1 3. Crystallizing ("ceramming") the glass article by an appropriate thermal treatment.
The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range. This is followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei.
Crystallization of glasses in the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 composition field generally provides highly crystallized glass-ceramics. The primary crystal phase may be a transparent beta-quartz solid solution, or an opaque beta-spodumene solid solution, depending on ceramming temperature.
Beta-quartz is the hexagonal trapezohedral modification of SiO.sub.2. It exhibits a slightly negative coefficient of thermal expansion (CTE). This makes it of particular interest where thermal cycling occurs, as in cookware. The basis of the beta-quartz solid solution is believed to be the substitution of Al.sup.+3 ions for some of the Si.sup.+4 ions in the beta-quartz structure. The attendant charge deficiency is made up by the introduction of a small ion, such as Li.sup.+, Mg.sup.+2, or Zn.sup.+2, into the beta-quartz structure.
Beta-quartz solid solution glass-ceramics customarily contain TiO.sub.2 as a nucleating agent. Optionally, the TiO.sub.2 may be partially, or wholly, substituted for by ZrO.sub.2.
The appearance of such Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 glass-ceramics can be varied by varying ceramming conditions, in particular heat treatment. Thus, transparent, translucent, or opaque glass-ceramics (which may be water-white, translucent, opaque white, or variously colored) are all possibilities.
The beta-quartz crystals in a transparent glass-ceramic are necessarily small in size. They are produced by ceramming the precursor glass at a relatively low temperature that normally does not exceed about 900.degree. C. If the same glass is cerammed at a higher top temperature on the order of 1150.degree. C., the opaque beta-spodumene crystal phase is produced. At such higher temperature, the small beta-quartz crystals convert to beta-spodumene and grow in size, thereby rendering the product opaque.
This flexibility is a very valuable attribute for at least two reasons. It permits producing both transparent and opaque ware from a single precursor glass. It also permits producing composition variants of the precursor glass, such as colorant variations, by using a forehearth additive system as later described in more detail. This facilitates producing more than one product from a single melting unit.
The widest use of Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 glass-ceramic materials has been in the field of culinary ware. For over three decades, Corning Glass Works, now Corning Incorporated, has marketed opaque white cooking utensils under the trademark CORNING WARE.
More recently, cooking utensils, formed from a transparent glass-ceramic exhibiting a light brown tint, were introduced commercially under the trademark VISIONS. In general, this transparent glass-ceramic is crystallized at lower temperatures to develop small, beta-quartz solid solution crystals.
It has been observed that transparent, beta-quartz glass-ceramics nucleated with TiO.sub.2 tend to exhibit a light brown tint. This is ascribed to the presence of both TiO.sub.2 and Fe.sub.2 O.sub.3 in the parent glass composition. Efforts to achieve a different color, then, must take into consideration this inherent coloration effect.
U.S. Pat. No. 5,070,045 (Comte et al.) discloses transparent, glass-ceramic plates wherein the predominant crystal phase in the glass-ceramics is beta-quartz solid solution. These plates use 0.1-1.0% of a colorant selected from CoO, NiO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO.sub.2, and V.sub.2 O.sub.5. The patent is primarily concerned with V.sub.2 O.sub.5 which contributes to minimal distortion while giving a black aspect in reflection and a reddish brown tint in transmission. The Comte et al. compositions consist essentially, in weight percent, as calculated on the oxide basis, of:
______________________________________ SiO.sub.2 65-70 MgO + BaO + SrO 1.1-2.3 Al.sub.2 O.sub.3 18-19.8 ZrO.sub.2 1.0-2.5 Li.sub.2 O 2.5-3.8 As.sub.2 O.sub.3 0-1.5 MgO 0.55-1.5 Sb.sub.2 O.sub.3 0-1.5 ZnO 1.2-2.8 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0.5-1.5 TiO.sub.2 1.8-3.2 Na.sub.2 O 0-&lt;1.0 BaO 0-1.4 K.sub.2 O 0-&lt;1.0 SrO 0-1.4 Na.sub.2 O + K.sub.2 O 0-&lt;1.0 BaO + SrO 0.4-1.4 2.8 Li.sub.2 O + 1.2 ZnO &gt;1.8 5.2 MgO ______________________________________
U.S. Pat. No. 5,179,045 (Aitken et al.) describes production of a burgundy color in a glass-ceramic having as its primary crystal phase a beta-quartz solid solution. The glass-ceramic contains up to 6% TiO.sub.2 as a nucleating agent. It has a color package composed of 50-150 ppm Co.sub.3 O.sub.4, 50-250 ppm NiO and 400-1000 ppm Fe.sub.2 O.sub.3 to provide the desired burgundy color.
U.S. Pat. No. 5,256,600 (Pfitzenmaier) describes a method of varying the color in a glass-ceramic material having a beta-quartz solid solution as the predominant crystal phase. The method comprises controlling the Al.sub.2 O.sub.3 level between 19 and 20% by weight, the Fe.sub.2 O.sub.3 level between 700 and 900 ppm and the Co.sub.3 O.sub.4 level at not over 15 ppm in the glass melt. Co.sub.3 O.sub.4 may be added to the molten glass in the forehearth to provide an amber color with 20-40 ppm Co.sub.3 O.sub.4 and a burgundy color with 120-140 ppm.
The present invention arose from a study directed at achieving greater utilization of the forehearth additive system in cookware design. In particular, the study was directed at determining what, if any, unique color effects might be obtained by varying the amount of cobalt oxide employed as a coloring additive in a precursor glass for Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 glass ceramics.